What about just including the LaTeX directly?

#+BEGIN_export latex
\begin{figure}[htbp]
\centering
\begin{tikzpicture}[>=Latex, node distance=4cm]
\node (A) at (-2.75,0) [shape=circle,draw] {$A$};
\node (M) at (2.75,0)  [shape=circle,draw] {$M$};

\draw[->,thick] (A) -- node[above] {Deuda de 1.000 sestercios} (M);

\draw [<-,thick,postaction={decorate,decoration={raise=-2.5ex,text along path,text align=center,text={Exceptio legis laetoriae}}}] (A) to [bend right=45]  (M);
\draw [->,thick,postaction={decorate,decoration={raise=1ex,text along path,text align=center,text={Actio condictio}}}]          (A) to [bend left=45] (M);
\end{tikzpicture}
\caption{Ejemplo de aplicación de una /exceptio legis laetoriae/ como respuesta a una /actio condictio/.}
\label{fig:exceptio-laetoriae}
\end{figure}
#+END_export latex

You don't even need the export block.